Subterranean deposits of coal, whether of "hard" coal such as anthracite or "soft" coal such as lignite or bituminous contain substantial quantities of methane gas entrained in the coal deposits. Limited production and use of methane gas from coal deposits has occurred for many years. However there are substantial obstacles which heretofore have frustrated more extensive development and use of methane gas deposits in coal seams. The foremost problem is the fact that coal seams, while they may extend over large areas of up to several thousand acres, typically are fairly shallow in depth, varying from a few inches to several meters. While they often are relatively near the surface (a thousand feet or less), vertical wells drilled into the coal deposits for obtaining methane gas can drain only a fairly small radius around the coal deposits. Further, the coal deposits are not amenable to pressure fracturing and other methods often used for increasing methane gas production from rock formations, so that, once the gas easily drained by a vertical well bore into the coal seam is produced, further production tends to be quite limited in volume. Additionally, coal seams often are associated with subterranean water, which must be drained from the coal seam in order to produce the methane.
Horizontal drilling patterns have been tried in order to extend the amount of coal seam exposed to a drill bore for gas extraction. But removal of the entrained water has presented difficulties in these operations. Horizontal drilling techniques require the use of a radiused well bore portion and a horizontal bore. The most efficient method for pumping water from a subterranean well, a sucker rod pump, does not work well in horizontal bores or around radiused bores.
A further problem which has been encountered in prior art techniques for producing gas from coal seams is the difficulty presented by under balanced drilling conditions resulting from the porousness of the coal seam. During the well drilling operations, whether vertical or horizontal, drilling fluid used to remove cuttings to the surface presents a hydrostatic pressure on the formation which, if it exceeds the hydrostatic pressure in the formation, can result in a loss of drilling fluid into the formation. This results in entrainment of drilling fines in the formation, which tends to plug up the small cracks and fractures which are needed to produce the gas.